This invention relates to the securing of heat sinks to electronic components.
In the electronics industry, printed circuit boards form parts of electronic circuits which include electronic components carried by the boards. These components generate heat during usage. Some components, e.g. integrated circuits, require heat to be removed from them to permit operation within acceptable operating ranges of temperature, i.e. temperatures not sufficiently high to render the components non-functional. In one method of removing heat, heat sinks are used. In use, a heat sink needs to be held in heat conductive relationship with a corresponding electronic component and a heat emitting projection or projections of the heat sink are located in a cooling air flow path for removal of the heat generated by the component.
Various types of design structures are employed for securing heat sinks with bases of the heat sinks in positions in heat conductive relationship with their electronic components. In a first conventional design structure for securing a heat sink in position, a clip having a leaf spring portion is positioned with the leaf spring portion extending across the base of the heat sink. Arms at opposite ends of the leaf spring portion extend down opposite edges of the base and around a substrate to one side of which the electronic component, in the form of an integrated circuit, is attached. A screw extends in screwthreaded engagement through the leaf spring portion of the clip substantially midway between its ends. The screw is tightened against the base of the heat sink to cause the leaf spring portion to flex away from the base thereby drawing the arms inwards towards the substrate. This action causes inwardly directed free ends of the arms to engage the side of the substrate remote from the electronic component. Increased screw tightening then results in tightly holding the substrate, the electronic component and the base of the heat sink between the ends of the arms and the screw.
A problem which exists in use of the above design structure is that the screw needs to have a certain minimal length for tightening and deforming the leaf spring portion of the clip which is flexed outwards from the base of the heat sink during the tightening operation. While the heat sink itself may be sufficiently small to fit comfortably together with the substrate and integrated circuit within narrow gaps (e.g. 9 mm to 10 mm), such as provided between adjacent printed circuit boards, the minimum length of screw prevents the screw from being accommodated in such a narrow gap. This clip structure also has the disadvantage that the leaf spring portion requires space to be positioned across the base, which space could more advantageously be occupied by a heat emitting projection or projections of the heat sink. As a result, optimal heat removal and dispersion cannot be achieved. Further, another reason for detracting from optimizing heat removal and dispersion is that the leaf spring portion of the clip is arched, in its tightened position, away from the heat sink base and extends along its length across cooling air flow passages between the heat emitting projections thereby effectively reducing flow passage area and also providing a resistance to the flow of cooling air.
In addition to this, the clip is formed from an electrically conductive material, i.e. a metal. In cases where the substrate has terminals or conductors on its side remote from the electronic component (particularly in the case of a ball grid array structure on the substrate), then the ends of the clip arms in engaging the remote side of the substrate are in danger of approaching too close to the terminals or conductors. Thus, shorting of the circuitry is a possibility.
Also, the screw incorporated in the above structure offers certain weaknesses in that a specific torque is required to tighten the clip to a desired degree to provide a required compressive contact between the base of the heat sink and the electronic component. Special torque applying tools may be available for this purpose. However, actual torque applied may be at the whim of the assembler who may not, in some cases, use a torque applying tool. It is also possible that a torque applying tool is incorrectly adjusted. If insufficient torque is provided during assembly, repeated temperature cycling in use, attended by shrinkage and expansion of materials, may result in loosening of the screw and slackening in compressive contact of the heat sink base with the electronic component. Vibration may also assist in loosening of the screw. In any event, loosening of the screw results in reduction in heat removal from the electronic component and could also result in complete detachment of the clip and heat sink from the component. Alternatively, the application of too much torque during assembly could increase strain on components, particularly during temperature cycling, and may result in cracking or breaking apart of the component, substrate, or heat sink.
In a second conventional design structure, a heat sink has a screwthreaded cylindrical metal base which is screwthreadedly attached through a hole in a plastic clip as a replacement for the screw and clip design discussed above. In this second structure, one end of the base is in heat conductive contact with its corresponding heat generating electronic component. At the other end of the base, the heat sink has a heat emitting projection in the form of a radially extending heat emitting flange. This latter design is shorter than the screw and clip of the first conventional design structure discussed above and may be suitably located in narrow gaps such as those referred to above. However, the heat emitting flange needs a specific dimension radially of the screwthreaded base in order to transfer the required heat from the electronic component into the cooling air flow. Unfortunately, this results in the flange overshadowing and extending beyond the boundaries of an electronic component for which it is suitably designed. Hence, when in use, for instance upon a printed circuit board, the flange also overshadows areas of the board which could otherwise be used for circuitry or attachment of circuitry components. Thus, freedom for circuitry design becomes limited.
In addition, the screwthreaded base of the heat sink is substantially large. This results in the length of the plastic clip on either side of the base being short and limited in flexibility. This may prevent the clip from being flexed to allow the arms of the clip to move apart sufficiently to pass down opposite edges of the heat sink and substrate to allow for engagement of the inwardly extending free ends of the arms with the remote side of the substrate for holding purposes. In this case the clip needs to be slid laterally onto the substrate during assembly. To achieve this in practice, i.e. with the component and its mounting substrate upon a printed circuit board, the board needs to be free of circuitry elements adjacent an edge of the substrate which first receives the clip. This provides a further limitation to freedom for circuitry design upon the printed circuit board. Also, because of the need for lateral application of the clip, positioning of the clip upon the electronic component relies entirely upon the judgment of the assembler. If the clip, and thus the heat sink, are not correctly positioned, this could reduce heat removal from the component.
In addition, while the latter heat sink and clip design may be located within minimally narrow gaps, the heat emitting flange may not provide sufficient distance from an adjacent article (e.g. printed circuit board) to ensure an adequate cooling air flow across the flange for the required cooling purposes. Further to this, the clip of this second conventional design structure provides cooling air flow passage obstruction problems and potential screw loosening problems similar to those discussed above with regard to the first conventional design structure. In the second design structure, plastic creep and stress relaxation also increase the possibility of screw loosening.
The present invention seeks to provide a clip for securing a heat sink to a heat generating electronic component and also a clip, heat sink and electronic component combination in which at least some of the above disadvantages are reduced or minimized.
According to one broad aspect, the invention provides a clip for holding a heat sink in heat conductive relationship with an electronic component mounted upon a first side of a substrate, the clip comprising a clip body and spring means, the clip body comprising:
a resilient beam for location in a position on one side of the heat sink; and
two arms extending in opposition from the beam in spaced positions along the beam, each arm having a free end with an electrically non-conductive free end extension which extends towards the other arm;
the clip body being resiliently flexible to move the free ends of the arms further apart for reception of the heat sink and the substrate between them and locate the free end extensions in positions opposing a second side of the substrate remote from the first side; and
the spring means is operably engageable with the resilient beam in a position between the arms to act between the beam and a surface of the heat sink so as to resiliently flex the beam away from the heat sink and pivot the arms to move their free end extensions towards each other on the second side of the substrate to compressively hold the heat sink, electronic component and substrate between the spring means and the free end extensions of the arms.
Clips according to the invention have certain distinct advantages.
In a case where an inventive clip together with a heat sink is used with a specific integrated circuit on a substrate, the clip and heat sink may lie substantially within the boundary lines of the substrate with the heat sink and substrate lying between the arms. Also, the clip may be applied directly down towards a side of the electronic component thus obviating the need for lateral sliding movement of the clip and the heat sink onto the component and substrate. Hence, it follows that a printed circuit board upon which the substrate is mounted may have circuitry including electronic components located in regions of the board adjacent to the substrate as the clip and heat sink will not overshadow these adjacent regions. Further, because the spring means is locatable between the beam of the clip and a surface of the heat sink, then the beam is located furthest from the heat sink. Hence, the beam may be positioned outwardly beyond any heat emitting projections of the heat sink or may overlie free end extremities of such projections. Thus the beam will not obstruct the most effective heat transfer parts of the seat sink, i.e. the parts of the projections which lie closest to a base of the heat sink and which become hotter in use than the extremities.
As the free end extensions of the arms of the clip have electrically insulated surfaces, then shorting of electrical terminals on the second side of the substrate by close proximity with these surfaces is avoided.
A clip of the invention, in having spring means, avoids the use of a screwthread attachment, thereby also avoiding loosening of the clip, for instance as caused by temperature cycling which may undo the screw. Also, clips of the invention may be designed suitable to provide a combined assembly height with heat sink, electronic component and substrate, to fit comfortably within narrow gaps, e.g. 9 mm to 10 mm, while having negligible or no effect upon flow of cooling air through passages between the projections of the heat sink.
In a preferred arrangement, the clip is formed with integral beam and arms and is made entirely from an electrically insulating material, i.e. a suitable plastic. Such a plastic is a polyethylerimide. However, should creep occur in the plastic, the spring means will compensate for this and maintain the required degree of compressive contact between the heat sink and the electronic component with no loosening effect. Also, the spring means has a spring rate which is predetermined by design to suit its function. Hence, compressive force of contact between the heat sink and electronic component is as required to optimize heat exchange. Continuous pressure is applied during dimensional changes due to temperature changes and the effects of different thermal coefficients of expansion. Hence the need for specific torque in tightening of a screw or as decided at the whim of the assembler is avoided.
With the arms of the clip locating the heat sink and substrate between them and with the resilient beam located at or outwardly of the free extremities of the heat emitting projections, the beam and arms define boundaries to air flow passages provided between the projections. Leading edges of the beam and arms (i.e. in the direction of movement of cooling air) may suitably be oriented to encourage or direct the air into the flow passages.
It is also preferable that the metal spring means comprises at least one compression spring which is locatable with its ends engaged with the resilient beam and with the heat sink. Other forms of spring means may be used, for instance a leaf spring, with the disadvantage that it may obscure air flow passages. The compression spring, however, may be located in a position in alignment with heat emitting projections of the heat sink so as to provide negligible or no obstruction to the air flow passages.
It is also preferable that a surface of the resilient beam, which faces away from and remote from the arms, is concave in an unstressed condition of the beam. Resilient flexing of the beam in use will tend to reduce the concavity of this surface and preferably without the surface increasing the effective height of the clip.
The invention further includes a combination of a substrate, an electronic component mounted upon a first side of the substrate, a heat sink and a clip for holding the heat sink in heat conductive relationship with the electronic component. The clip is as defined according to the invention above.